Implantable tissue stimulation systems have proven therapeutic in a wide variety of diseases and disorders. Pacemakers and Implantable Cardiac Defibrillators (ICDs) have proven highly effective in the treatment of a number of cardiac conditions (e.g., arrhythmias). Spinal Cord Stimulation (SCS) systems have long been accepted as a therapeutic modality for the treatment of chronic pain syndromes, and the application of tissue stimulation has begun to expand to additional applications such as angina pectoralis and incontinence. Deep Brain Stimulation (DBS) has also been applied therapeutically for well over a decade for the treatment of refractory chronic pain syndromes, and DBS has also recently been applied in additional areas such as movement disorders and epilepsy. Further, Functional Electrical Stimulation (FES) systems such as the Freehand system by NeuroControl (Cleveland, Ohio) have been applied to restore some functionality to paralyzed extremities in spinal cord injury patients. Furthermore, in recent investigations Peripheral Nerve Stimulation (PNS) systems have demonstrated efficacy in the treatment of chronic pain syndromes and incontinence, and a number of additional applications are currently under investigation. Occipital Nerve Stimulation (ONS), in which leads are implanted in the tissue over the occipital nerves, has shown promise as a treatment for various headaches, including migraine headaches, cluster headaches, and cervicogenic headaches.
These implantable tissue stimulation systems typically include one or more electrode carrying stimulation leads, which are implanted at the desired stimulation site, and a neurostimulator (e.g., an implantable pulse generator (IPG)) implanted remotely from the stimulation site, but coupled either directly to the tissue stimulation lead(s) or indirectly to the tissue stimulation lead(s) via a lead extension. Thus, electrical pulses can be delivered from the neurostimulator to the tissue stimulation leads to stimulate the tissue and provide the desired efficacious therapy to the patient. The tissue stimulation system may further comprise a handheld patient programmer in the form of a remote control (RC) to remotely instruct the neurostimulator to generate electrical stimulation pulses in accordance with selected stimulation parameters. A typical stimulation parameter set may include the electrodes that are acting as anodes or cathodes, as well as the amplitude, duration, and rate of the stimulation pulses. The RC may, itself, be programmed by a clinician, for example, by using a clinician's programmer (CP), which typically includes a general purpose computer, such as a laptop, with a programming software package installed thereon. Typically, the RC can only control the neurostimulator in a limited manner (e.g., by only selecting a program or adjusting the pulse amplitude or pulse width), whereas the CP can be used to control all of the stimulation parameters, including which electrodes are cathodes or anodes.
In the context of an SCS procedure, one or more stimulation leads are introduced through the patient's back into the epidural space, such that the electrodes carried by the leads are arranged in a desired pattern and spacing to create an electrode array. One type of commercially available stimulation leads is a percutaneous lead, which comprises a cylindrical body with ring electrodes, and can be introduced into contact with the affected spinal tissue through a Touhy-like needle, which passes through the skin, between the desired vertebrae, and into the epidural space above the dura layer. For unilateral pain, a percutaneous lead is placed on the corresponding lateral side of the spinal cord. For bilateral pain, a percutaneous lead is placed down the midline of the spinal cord, or two or more percutaneous leads are placed down the respective sides of the midline of the spinal cord, and if a third lead is used, down the midline of the spinal cord. After proper placement of the tissue stimulation leads at the target area of the spinal cord, the leads are anchored in place at an exit site to prevent movement of the tissue stimulation leads. To facilitate the location of the neurostimulator away from the exit point of the tissue stimulation leads, lead extensions are sometimes used.
The tissue stimulation leads, or the lead extensions, are then connected to the IPG, which can then be operated to generate electrical pulses that are delivered, through the electrodes, to the targeted tissue, and in particular, the dorsal column and dorsal root fibers within the spinal cord. The stimulation creates the sensation known as paresthesia, which can be characterized as an alternative sensation that replaces the pain signals sensed by the patient. Intra-operatively (i.e., during the surgical procedure), the neurostimulator may be operated to test the effect of stimulation and adjust the parameters of the stimulation for optimal pain relief. The patient may provide verbal feedback regarding the presence of paresthesia over the pain area, and based on this feedback, the lead positions may be adjusted and re-anchored if necessary. A computer program, such as Bionic Navigator®, available from Boston Scientific Neuromodulation Corporation, can be incorporated in a clinician's programmer (CP) (briefly discussed above) to facilitate selection of the stimulation parameters. Any incisions are then closed to fully implant the system. Post-operatively (i.e., after the surgical procedure has been completed), a clinician can adjust the stimulation parameters using the computerized programming system to re-optimize the therapy.
The efficacy of SCS is related to the ability to stimulate the spinal cord tissue corresponding to evoked paresthesia in the region of the body where the patient experiences pain. Thus, the working clinical paradigm is that achievement of an effective result from SCS depends on the tissue stimulation lead or leads being placed in a location (both longitudinal and lateral) relative to the spinal tissue such that the electrical stimulation will induce paresthesia located in approximately the same place in the patient's body as the pain (i.e., the target of treatment). If a lead is not correctly positioned, it is possible that the patient will receive little or no benefit from an implanted SCS system. Thus, correct lead placement can mean the difference between effective and ineffective pain therapy, and as such, precise positioning of the leads proximal to the targets of stimulation is critical to the success of the therapy.
One important parameter that can influence the electrical field generated by stimulation electrodes, and thus the proper placement of the leads, is the electrical conductivity environment due to the tissue characteristics surrounding the electrodes. This is often a secondary concern to the clinician, who will typically place the tissue stimulation leads based on anatomic (fluoroscopy, ultrasound, etc.) and physiologic (compound action potentials, EMG, etc.) landmarks, and then anticipate that the electrical environment seen by the electrodes will promote good coupling efficiency between the tissue stimulation leads and the target tissue, and will therefore not significantly affect the stimulation therapy.
However, it cannot always be assumed that the coupling efficiency is at a level high enough to achieve optimum performance even when the position of the tissue stimulation leads relative to the tissue appears to promote good coupling efficiency under conventional imaging. For example, in the case of SCS, when the tissue stimulation leads are viewed in the epidural space of the patient under a conventional anterior fluoroscopic image, the leads may appear properly located relative to the spinal cord. In reality, however, portions of the tissue stimulation leads may be dorsally located from the spinal cord a relatively far distance, which will not be appreciated from a conventional anterior fluoroscopic image. As a result, the coupling efficiency between the electrodes of the tissue stimulation leads that are relatively far from the spinal cord may be quite low, which may adversely affect the performance of the tissue stimulation system.
For example, in single-source tissue stimulation systems, the impedance seen at each electrode may influence the amount of electrical current that can be delivered from each electrode, and thereby shape the electrical field in a non-controllable manner. If the impedance is high enough, coupling efficiency between the electrodes and the target tissue to be stimulated will be so low that stimulation performance will be significantly degraded. Even for multi-source tissue stimulation systems that precisely control the magnitude of electrical current at each electrode, the occurrence of a low coupling efficiency between the electrodes and the surrounding tissue due to high electrode impedance, forces the system to use more energy that what would otherwise be necessary to maintain stimulation performance. As a result, an excessive amount of compliance voltage may need to be generated in order to effectively supply stimulation energy to the electrodes if the tissue stimulation system uses current-controlled sources, thereby resulting in an inefficient use of the battery power, or the stimulation energy supplied to the electrode may be otherwise inadequate if the tissue stimulation system uses voltage-controlled sources.
There, thus, remains a need for a system and method for positioning tissue stimulation leads within a tissue region of the patient that provides a suitable electrical conductivity environment for optimizing the conveyance of electrical stimulation energy from the tissue stimulation leads.